Field of the Invention
The present invention relates to a sensor comprising a magnet and a Hall-effect probe and intended to detect if a mobile metal object is in front of the Hall-effect probe, as well as a magnet for such a sensor.
Description of Related Art
To check the presence of a mobile metal object, it is known to use a sensor comprising a magnet coupled to a Hall-effect probe.
Such a sensor is used for example to detect the position of the axis of the dog clutch of a gearbox, or else the position of the camshaft. FIG. 1 shows a curve 10 that represents the variations in magnetic induction as a function of the position of the mobile object with respect to the Hall-effect probe. The value of the magnetic induction decreases when the mobile object moves away from the Hall-effect probe.
Currently, manufacturers in particular of motor vehicles, require that the position of the mobile object with respect to the Hall-effect probe be determined from the start-up of the sensor. This function is called TPO (True Power On.)
This function thus makes it possible to determine at start-up whether or not the mobile object is facing the Hall-effect probe. To do this, a magnetic induction threshold value 12 is set in the factory.
Thus, when the sensor starts up, if the value of the magnetic induction seen by the Hall-effect probe is above the threshold value 12, this is interpreted as the fact that the mobile object is facing the Hall-effect probe. If, on the contrary, when the sensor starts up, the value of the magnetic induction seen by the Hall-effect probe is below the threshold value 12, this is interpreted as the fact that the mobile object is not facing the Hall-effect probe.
Over the lifetime of the sensor, it can happen that the Hall-effect probe moves with respect to the magnet and then the magnetic induction value changes. The curve 14 is the curve representing the variations in magnetic induction after movement of the Hall-effect probe.
As can be seen in FIG. 1, the threshold value 12 is no longer reached after the movement of the Hall-effect probe and the TPO function is no longer fulfilled.
To solve this problem, it is known practice to fasten the Hall-effect probe in the sensor and therefore to make the Hall-effect probe immobile with respect to the magnet. The fastening is performed for example by casting resin in the sensor, thereby encircling the Hall-effect probe.
Such a solution has other disadvantages. For example, the fastening of the Hall-effect probe leads to the appearance of mechanical stresses due to the thermal expansions in the Hall-effect probe and in particular at the level of the welds. Furthermore, the placing of the resin is burdensome and requires the use of an appropriate machine.
From the document U.S. Pat. No. 5,637,995 a sensor is known composed of a probe including a sensitive element, a permanent magnet, a casing and a hood. The permanent magnet has a recess and a through hole running along the central axis of the permanent magnet. The through hole communicates with the recess. The probe runs along the through hole and into the recess, the sensitive element being housed at the level of the recess. The permanent magnet and the probe are held in place by the casing and the hood. The probe is held in place in the magnet in order to come permanently in contact with the hood.